Encyclopaedia Britannica > Volume 3, Anatomy-Astronomy

ANATOMY.
25
Special be observed, consist of a peculiar form of cancellated
Anatomy, structure. Externally their bony walls are perforated by
numerous minute well-defined holes, into which, in the
The lower recent state, the vessels and filaments of the alveolar pe-
^aW' riosteum penetrate. These orifices are so numerous in
both maxillary bones, that a portion of alveolar process
held between the eye and the light seems entirely per¬
forated, and, if injected, becomes completely red. This
character applies chiefly to the period of youth and ado¬
lescence. After this the alveolar processes become more
solid, and eventually present much fewer orifices. This
structure is the result of the manner in which the bony
matter is deposited round the vessels and filaments of the
periosteum, which may in this and similar bones be said
to communicate in this manner with those of the medul¬
lary membrane.
The Teeth. (Dentes.)
The teeth. The teeth, which are implanted in the alveolar cavi¬
ties, though belonging properly to the digestive apparatus
as organs of mastication, are nevertheless more conve¬
niently considered in this place. (Plate XXVI. fig. 7, 8,
and 9.)
In every tooth are recognised three parts,—the crown
(corona), the neck or collar (collum), and the root (radix).
The first is the portion which appears above the gum and
alveolar process, and consists of a layer of enamel, thick
above and round the top, but gradually extenuated below,
inclosing a small portion of compact bone. The crown
may be distinguished into the crown proper, or the sum¬
mit or apex of the tooth, and the fillet or annular portion
of enamel (annulus) by which the sides are surrounded.
The root, which consists of compact bone, is the part
which is implanted in the alveolar cavity, and the outer
surface of which adheres to the inner or dental surface of
the alveolar periosteum, and above that to the soft part
of the gum. The neck is the narrow ring where the ena¬
mel ceases and the bone begins, and to which the gum
adheres all round. This part, in short, belongs neither
to the crown nor to the root, and perhaps is not justly
entitled to any separate consideration, unless as the por¬
tion to which the gum adheres. The crown and root, on
the contrary, are important, as furnishing the characters
by which the teeth are distinguished into classes.
Theincisor These are three,—the incisor or cutting teeth (dentes
tomici, incisores, primores, risorii, acuti, adversi); the ca¬
nine or tearing teeth (dentes canini, laniarii, cuspidati);
and the molar or grinding teeth (dentes molares).
The incisors are in number eight, four in each jaw, and
two on each side of the mesial plane. All of them agree
in having the crown or apex wedge-shaped, or like a car¬
penter’s adze, convex before, and slightly concave be¬
hind, with the cutting edge crenated or notched. From
the edge, also, which is broad, the fillet, which is quadri¬
lateral, contracts much to the neck, and the root is be¬
velled in the opposite direction to that of the crown, so
as to form a quadrilateral prism, and slightly acuminated.
It is always simple and elongated. Its extremity presents
an orifice communicating with the interior of the tooth,
and admitting the vessels by which it is nourished. The
crown is separated from the root by a narrow line, vary¬
ing in position in the different kinds of incisors.
The incisors of the upper jaw are broader, thicker,
longer, and in general more powerful, than those of the
lower, and their axes are directed downwards and for¬
wards so as to overlap in the motions of the lower jaw
those of the inferior row, and leave a triangular interval
with the base upwards. Their cutting edge also is ob¬
lique, so that its mesial angle is longer than its external;
vol. m.
teeth.
and their roots are larger and rounder. The central or Special
middle incisors are larger, broader, and stronger, than the Anatomy,
lateral ones.
The incisors of the lower jaw are smaller than those of caA
the upper, and their axes are directed upwards and back- 1 6 *
wards, so that they are within the superior incisors. The
central incisors also are larger than the lateral ones.
The canine or pointed teeth (dentes cuspidati) are four
in number, one in each half of each alveolar arch, imme¬
diately next to the lateral incisors. These teeth are
distinguished by prominent, thick crowns, with rounded
convex fillets and pointed apices. The root also, which,
like that of the incisors, is simple, is larger and thicker
than in the latter. The upper canine teeth, which occa¬
sionally are named eye-teeth (ocularii dentes), are the
longest and most prominent of all. During the elevation
of the lower jaw, each inferior canine tooth is interior to
the superior one, and is interposed between it and the
lateral incisor.
Of the molar or grinders there are 20 altogether, being
five in each half of each alveolar arch. They are sub¬
divided into two orders, small molar or bicuspid teeth
(dentes bicuspidati), and large molar or multicuspid teeth
(dentes multicuspidati).
The molar teeth agree in having large, broad crowns, The molar
with tubercular summits, and roots, which, though occa-or grinding
sionally single, are much more frequently two-fold, three-teet^‘
fold, or four-fold. The coronal tubercles are generally
parted by a deep furrow, which gradually becomes shallow
as the summits wear in the progress of years. When the
roots are single, which is most frequent in the bicuspid
order, each lateral surface is marked by a longitudinal
furrow. When two-fold or manifold, they are parted by a
fissure of variable width; and sometimes they diverge
considerably. The annular furrow between the crown
and root is very distinct.
The axes of the upper molar teeth are directed out¬
wards, while those of the lower order, especially the most
posterior, are directed inwards. More anteriorly they
are occasionally vertical.
The two molar teeth next to the canine, which have The small
smaller crowns and roots, and are less in all their di-™^ar.or
mensions than the three posterior ones, are therefore dis-^”^1
tinguished as small molars. From the circumstance of
their crowns being provided with two conical apices, one
anterior, large, and the other posterior, parted by a trans¬
verse furrow, not dissimilar to the summits of two canine
teeth conjoined in one, they were denominated by John
Hunter bicuspid teeth (bicuspidati). These teeth are
smaller than the canine, especially when their roots are
single. The roots, however, are often bifid, and some¬
times trifid. In the first case they are bevelled on the
sides, and terminate in a point. In the second and third
case they are conical. The internal surface of the bicus¬
pid teeth is very generally narrower than the external, so
that a transverse section would be trapezoidal, with the
narrow margin posterior.
The third molar tooth, or the first large molar, is gene- The larger
rally very large and strong, with the crown broad and qua- ™ ”gr
drilateral, surmounted by four, five, or more tubercles, and proper.
the root trifid or quadrifid and divergent in the upper
tooth, more frequently bifid only and divergent in the
lower. The second large molar tooth, though rarely so
large as the first, has the same general shape; the crown
rhomboidal in the upper jaw, with four apices, and the
root bifid or trifid and divergent.
The third large molar, named also wisdom-tooth (dens
sapientice), from its late appearance, is smaller than the
second, generally has a narrower crown, rounded, oblong,
D